Optical scanners

Abstract

An optical scanner with plural scan beam generation. Scan angle stress is configured in the beam generation.

Description

BACKGROUND OF THE INVENTIONThis invention relates to optical scanners and signal processors used therein and more particularly to bar code scanners having multi-bit digitizers used to detect a pattern of optically reflective / non reflective white space / black bar indicia and provide multi-bit digital signal representations of such indicia.The invention further relates to a method of scanning indicia using selective sampling, particularly although not exclusively to a method of reading bar codes using a laser scanner. The invention further relates to a digitizer, for example a non-linear edge strength digitizer, to a signal processing apparatus in particular such an apparatus including a matched filter for bar code symbol reading, and to a signal processing apparatus, in particular such an apparatus including an automatic deblurring arrangement.As is known in the art, optical scanners and signal processors used therein, have a wide range of applications. One such application is in read...

AI Technical Summary

Benefits of technology

With such an arrangement, the requirement of relatively expensive analog-to-digital converters is removed. Further, the interface between the digitizer and the decoder requires fewer lines.

The method of the present invention allows aggressive and reliable bar code symbol decoding, after the optical detection system in the scanner has transduced it into a distorted analog waveform.

If an attempted decode fails, based upon the samples that have been obtained, mechanical, electrical or logical adaptations are made to the scanner in an attempt to provide an improved decode on the next scan. To that end, the decoder analyses the samples to provide information on the extent to which the samples appear to be represent data corresponding to the indicia, and the extent to which the samples are spurious (for example because they represent noise). The information thus determined is used to provide feed-back to adapt the scanner to the current scanning environment, thereby providing improved performance on the next subsequent scan.

A timing signal is preferably produced which synchronizes and / or provides timing information on the samples that have been taken. In the preferred embodiment, the timing signal is a square wave which changes state whenever a sample has been taken. The timing signal provides further information which is used by the decoder, in association with the values of the samples themselves, thereby enabling the decoder to attempt to decode the indicia.

The method of the present invention may be particularly useful in devices which make use of non-conventional optics, for example axicon or holographic optics. Such optics may increase the working range of a laser scanner significantly, as is described for example in U.S. Pat. No. 5,080,456. The profile produced by such non-conventional optics are multi-modal and contain side lobes. These side lobes introduce additional "wiggles" or bumps on the analog signal. These additional "wiggles", if detected by a standard digitizer, will most probably result in an unrecoverable error in the bar code representation, thereby making the system less reliable. However, if such non-conventional optics are used in conjunction with the method of the present invention, a much more robust system can be created. Small wiggles introduced by the scanning beam profile may be ignored by the decoder, merely by properly selecting the larger features and ignoring the smaller ones.

The method of the present invention is also particularly useful in scanners that make use of enhancement filters to increase the depth of modulation, for example as described in U.S. Pat. No. 5,140,146. In such devices, the resulting enhanced analog signal normally exhibits "ringing". This ringing introduces additional "wiggles" in the analog signal, similar to those described above. However, if enhancement filters are used in conjunction with the method of the present invention, a much more robust system can be created. Small wiggles introduced by the enhancement filter can be ignored by the decoder by properly selecting the larger features and ignoring the smaller ones. This also reduces the trade-off that has to be made between reading high density bar code symbols with a very low depth of modulation, and low quality bar code symbols that might include printing noise (for example dot matrix bar codes). Aspects of the invention are claimed in the independent claims attached hereto, and preferred features are claimed in dependent claims attached hereto.

Problems solved by technology

This simple digital representation of the data works extremely well in many situations, although it may sometimes be susceptible to unrecoverable errors if the bar code symbol to be read has substantial noise associated with it.

With this prior art representation, a single extra edge detected or shifted due to noise may prevent proper decoding.

This solution is, however, very expensive due to the large amount of samples required and the high speed processing that is necessary.

In particular the decoder cannot analyze the signal applying different noise thresholds on the same scan data stream.

A problem in known systems is that of detecting the edges of transitions whilst simultaneously suppressing the noise in the signal.

While solutions to this problem have been proposed in the communication industry, those solutions cannot be applied directly to the reading of printed indicia such as bar code symbols.

Furthermore, in communication systems a bit synchronisation clock is available or can be extracted for determining when to sample the output of a filter, but in a bar code scanning system such a clock is not available and cannot be extracted as the scan speed is never constant.

If the desired reading distance changes, therefore, this filtering effect makes edge detection difficult and erroneous and it is desired to solve that problem.

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